JP4568372B2 - Partial coherence interferometer eliminating measurement ambiguity - Google Patents

Abstract

A partial coherence interferometer incorporates a focusing system for resolving measurement ambiguities. A focus-sensing beam is directed through a common objective with the measurement beam of the interferometer for conveying the beams to and from a test surface. An unambiguous measuring range is equated to a predetermined range of focusing errors.

Description

  The present invention relates to the field of partial coherence interferometry, and to the resolution of ambiguities associated with interferometric measurements across zero states.

  Partial coherence interferometers measure distance by monitoring interference as a function of wavelength. The measurement beam has a spectral bandwidth over the wavelength range and is temporarily separated into a test beam and a reference beam. The test beam propagates along the test arm, encounters the test surface along the way, and reaches the detector. The reference beam propagates along the reference arm, encounters the reference plane along the way, and reaches the same detector. Height variations at different points on the test surface result in corresponding changes in the optical path length of the test arm.

  The detector decomposes the combined beam of the test beam and the reference beam into common spectral components and detects the intensity variation associated with interference for each sample wavelength. Interference as a function of wavelength is related to the optical path length difference between the test beam and the reference beam. When the optical path lengths of the test arm and the reference arm are equal, a so-called “null condition” occurs. Regardless of whether the test arm is longer than the reference arm or whether the reference arm is longer than the test arm, the period of maximum and minimum interference increases on both sides of the zero state.

  For this reason, partial coherence interferometers are generally configured to avoid a zero state. However, when used near the zero state, the partial coherence interferometer must also be configured to distinguish between cases where the optical path length of the test arm is longer than the optical path length of the reference arm. Distance sensors, especially triangulation distance sensors, have been used to identify clear measurements within a limited range. However, such distance sensors are generally not suitable for higher resolution measurements over a wider area, and are irregular surfaces, i.e. measuring surfaces such as tool traces and other artificially rough surfaces. It is more likely to be disturbed by a surface having a roughness that is at least one-eighth the wavelength of the beam.

  The present invention relates to a partial coherence interferometer, and assumes an interferometer for measuring the surface shape of a test object having a measurement range of, for example, about 1 millimeter. Focus sensors that can operate in longer measurement ranges, for example 3-5 millimeters, have their measurements matched to the zero state of the interferometer, so that measurements in the clear range are close to or opposite the zero state. Distinguish from other measurements that spread to the side. It is possible to measure the test surface over a large working distance with low beam shadowing. This surface to be tested has various reflectivity, for example, 100% to less than 1% reflectivity through diffuse reflection including regular reflection.

  The proposed distance sensor system includes a focus sensor, shares a part of the test arm of the partial coherence interferometer, directs the light to the test object surface via the shared objective lens, and from the test object. Oriented to go back. Preferably, the objective lens has a numerical aperture as low as 0.1 and can handle a large working distance. The focus sensor system can share part of the measurement beam, or it can supply its own sensor beam, especially for operation within a range of different wavelengths.

  By obscuring about half of the focus sensor beam at the focus position of the sensor lens, a change in the axial position of the test surface encountered by the focus sensor beam causes the detectable lateral distribution of the light distribution back to the focus detector. Converted to displacement. The zero state of the interferometer can be calibrated by the lateral displacement of these light distributions, so that a predetermined range of lateral displacement corresponds to a measurement range offset from one side of the zero state. The focal position of the interferometer that has passed through the objective lens is preferably set so that the optical path length difference between the test arm and the reference arm is equal to about 60 percent of the measurement range (eg, 0.60 × 1 mm = 0.6 mm). Is done. Therefore, at the end closest to the zero state in the measurement range, the test target surface is still offset from the zero state by a predetermined amount (for example, 0.1 mm). The measured value corresponding to the axial displacement from the focal position beyond the minimum offset from the zero state is adopted for measurement. On the other hand, measured values that are closer to the zero state or correspond to axial displacements on the opposite side of the zero state are excluded or eliminated as measured values with different signs. This is necessary because partial coherence measurements are not accurate near the zero state.

  One aspect of the invention as a partial coherence interferometer employs a measurement beam having a predetermined spectral bandwidth. A test arm sends a test beam portion of the measurement beam along a path that encounters the test plane, and a reference arm sends a reference beam portion of the measurement beam along a path that encounters the reference plane. The test arm includes a focusing optical element for focusing the test beam on the test surface and collecting a reflected portion of the test beam from the test surface. A spectrally sensitive detection system coupled to the test arm and the reference arm provides interference as a function of wavelength over a range of measurement beam bandwidths, and an unsigned optical path length difference between the test arm and the reference arm. Monitor as measured value. A focus detection system calibrated with the interferometer identifies optical path length differences between the test arm and the reference arm that are greater than zero and optical path length differences that are less than zero. Resolve the ambiguity associated with unsigned optical path length difference measurements.

  The focus detection system preferably includes a focus detection arm that sends light reflected from the test surface through the focusing optical element to a focus detector. Further, the focus detection system preferably includes the position of the test surface where the optical path length difference between the test arm and the reference arm is greater than zero, and the optical path length difference between the test arm and the reference arm. And the position of the test surface where the value is smaller than zero. The focus detection system preferably also distinguishes between an optical path length difference close to zero and an optical path length difference of any sign that is significantly larger or smaller than zero.

  The processor preferably includes: (a) a measurement from the spectral sensitivity detection system associated with an unsigned optical path length difference between the test arm and the reference arm; and (b) the test surface is the test arm and the test arm. Information from the focus detection system indicating whether the optical path length difference from the reference arm is at a position where the optical path length difference is greater than zero or a position where the difference is smaller is received. The light reflected from the test surface, through the focusing optical element, and to the focus detector can be part of the measurement beam. Alternatively, the interferometer further comprises a first light source for generating the measurement beam and a second light source for generating a focus detection beam, the focus detection beam being focused by the focus detection arm. The optical element is sent to the test surface and from the test surface through the focusing optical element to the focus detector. Preferably, at least one display wavelength of the focus detection beam or the measurement beam is in a visible wavelength range, and a visible spot is presented on the test surface.

  The focus detection system can be calibrated with the interferometer, and this calibration is performed by focusing the light passing through the focusing optical element in a zero state where the optical path length of the test arm and the optical path length of the reference arm are equal. To position with a predetermined offset. Preferably, the predetermined offset is at least half of a predetermined measurement range of the interferometer.

  Another aspect of the invention is a method for resolving measurement ambiguity of partial coherence interferometry based on an unsigned optical path length difference between a test beam and a reference beam. A measurement beam having a predetermined spectral bandwidth is separated into a test beam propagating along the test arm and a reference beam propagating along the reference arm. The test beam is directed along the test arm through the focusing optical element to the test surface and through the focusing optical element back from the test surface. The test beam and the reference beam are recombined and the interference over the range of wavelengths of the recombined beam is monitored as a measurement of the unsigned optical path length difference between the test arm and the reference arm. Light reflected from the test surface is sent through the focusing optical element to a focus detector for measuring focus error. The measured focus error identifies that one of the optical path length of the test arm and the optical path length of the reference arm is significantly shorter or significantly longer than the other.

  The focus of the focusing optical element is preferably located at a position offset from a zero state where the optical path lengths of the test arm and the reference arm are equal. The position of the test surface where the optical path length difference between the test arm and the reference arm is significantly larger than zero, and the test surface where the optical path length difference between the test arm and the reference arm is significantly smaller than zero. Identify the location of At that time, preferably, the optical path length difference close to zero and the optical path length difference of any sign that is significantly larger or significantly smaller than zero are identified.

The focus error range preferably correlates with the optical path length difference range between the test arm and the reference arm, where the optical path length of the test arm is more significant than the optical path length of the reference arm. Either long or significantly shorter. A monitored interference measurement of the optical path length difference between the test arm and the reference arm is selected based on an association with the identified focus error range.

FIG. 3 is a partial coherence interferometer including a focus detection system with another light source. It is a graph which shows the 1st relationship between the zero state of an interferometer, and a focus error range. It is a graph which shows the 2nd relationship between the zero state of an interferometer, and a focus error range. 1 is a partial coherence interferometer including a focus detection system sharing a light source. FIG.

  As shown in FIG. 1, the partial coherence interferometer 10 includes a light source 12 for generating a measurement beam 14. This measuring beam has a predetermined spectral bandwidth, preferably in the visible spectrum or in the infrared spectrum. The light source 12 is a broadband light source such as a super-radiant light emitting diode. If the display wavelength is about 800 nanometers, the output is about 10 milliwatts and the spectral bandwidth is about 40 nanometers, usually 10 nanometers to 150 nanometers. Those within the range are preferred. Such a light source 12 can also be called a light source with low temporal coherence. The collimating lens 16 collimates the spread measurement beam 14 so that it can propagate through the interferometer 10.

  A 50/50 beam splitter 22 splits the measurement beam 14 into a test beam 26 and a reference beam 28. Test beam 26 passes through 50/50 beam splitter 22 and propagates along test arm 32 of interferometer 10. The reference beam 28 is reflected by the 50/50 beam splitter 22 and propagates along the reference arm 34 of the interferometer 10.

  The test beam 26 passes through a first wavelength-sensitive dichroic beam splitter 24, is reflected by a second wavelength-sensitive dichroic beam splitter 36, and passes through an objective lens 38. The objective lens 38 converges the test beam 26 toward a focal point 42 on or at least in the vicinity of the test object surface 40. The objective lens 38 collects light reflected by specular reflection, diffuse reflection, or some combination of specular reflection and diffuse reflection from the test object surface 40, and converts the test beam 26 on the return path into two dichroic beam splitters 36. , 24 and return to the 50/50 beam splitter 22. Preferably, the objective lens 38 has a numerical aperture of about 0.1 so that the working distance is about 70 millimeters, for example, and the recommended measurement range is about 1.0 millimeter.

  A portion of the return test beam 26 and the reference beam 28 are recombined at the 50/50 beam splitter 22 and propagate together through the focusing optical element 52 toward the spectrally sensitive detection system 54. That is, the 50/50 beam splitter 22 reflects a portion of the return test beam 26, passes a portion of the return reference beam 28, and directs it through the focusing optical element 52 in a common direction. The spectral sensitivity detection system 54 is preferably a spectrometer combining a diffraction grating 56 and a linear CCD (charge coupled device) sensor array 58. The diffraction grating 56 angularly separates the measurement beam (ie, the recombination of the test beam 26 and the reference beam 28) at different wavelengths. The linear CCD sensor array 58 measures the interference intensity of each spatially separated wavelength.

  A focusing system 60 is coupled to the interferometer 10 via a first dichroic beam splitter 24 to avoid measurement ambiguities associated with optical path length differences across or near the zero state. Another light source 62, preferably a light source such as a laser diode operating in the visible spectrum (eg, wavelength 650 nanometers) generates the focus detection beam 64. This focus detection beam 64 is collimated in a spread form by a collimating lens 66 and propagates through the interferometer 10 with the rest of the focusing system 60 as a collimated beam. The beam splitter 68 reflects the collimated focus detection beam 64 and follows the path to the dichroic beam splitter 24. The beam splitter 68 can be configured as a partially reflective or polarization-sensitive beam splitter. Due to the wavelength sensitivity of the dichroic beam splitter 24, the focus detection beam 64 is reflected along a common path with the test beam 26, passes through the objective lens 38, reaches the test target surface 40, and is reflected by the test target surface.

  A part of the focus detection beam 64 reflected by the test target surface 40 returns to the beam splitter 68 via the dichroic beam splitter 24, and further propagates through the focus detection optical element 72 to the focus detector 74. Using a quarter wave plate 76 with a beam splitter 68 configured as a polarizing beam splitter, the polarization of the focus detection beam 64 is rotated and the returned focus detection beam 64 is directed more efficiently to the focus detector 74. Can do. Alternatively, a less expensive 50/50 beam splitter may be used as the beam splitter 68, although the light efficiency is inferior, in which case a quarter wave plate is not required.

  Prior to installing the above-described offset stop 65 that blocks substantially half of the cross-section of the focus sensing beam 64, the focus detection optical element 72 preferably has the focus 42 of the objective lens 38 on the focus detector 74. Tie conjugate image 78. As the axial position of the test object surface 40 on the optical axis 80 of the objective lens 38 changes, the distribution of light between the two photodetectors in the image plane of the focus detector 74 changes.

2A and 2B are plots of the focus error range as the relationship between the difference in the output of the photodetector shown below and the sum over the range that can be taken by the position of the test target surface 40 along the optical axis.
Focus error = (A−B) / (A + B)
Here, “A” and “B” are the outputs of the two photodetectors in the image plane of the focus detector 74, respectively.

  In both FIG. 2A and FIG. 2B, the focal point 42 of the test beam and the focus detection beam is located in the middle of the intended measurement area 82, and the zero state 84 of the interferometer 10 is one end of the intended measurement area 82. It is in a position slightly beyond. For example, the optical path length of the reference arm 34 can be configured to be longer or shorter than the optical path length of the test arm 32 by about 60% of the length of the intended measurement area 82. Since the intended measurement area 82 is centered on the focal point 42, the null state 84 is outside the intended measurement area 82 by an offset equal to about 10 percent of the length of the intended measurement area 82. To position. Thus, if the measurement range extends over 1.0 millimeter, the focal point 42 is displaced from the zero state 84 by 0.6 millimeters, and the zero state 84 is off the measurement area 82 by 0.1 millimeter.

  In FIG. 2A, one end of the measurement area is located at the intersection 86 where A and B are equal, which is achieved by an offset in the position of the detector 74. Thereby, the focus error associated with the intended measurement area 82 takes a value between zero and a predetermined negative value. Other positions of the test surface 40 within the range where the zero state 84 and focus error are positive are excluded from the intended measurement area 82. In FIG. 2B, the focal point 42 is located at a focus error intersection 86 where A and B are equal. The boundary of the measurement area 82 is determined by the absolute value of the focus error signal.

An astigmatic focus sensor is also suitable for these purposes, but it is described in the book “Magnet-Optical Data Recording Handbook (1997)” published by Neues Publishing Company, “Handbook of Magneto-Optical Data Recording”. ”, Noyes Publications), P.100-102. A quadrant detector with output signals A, B, C, and D is used for a focus detector based on this astigmatism method. The standardized focus error signal from this detector is expressed as:
Focus error = {(A + C)-(B + D)} / (A + B + C + D)
Here, “A”, “B”, “C”, and “D” are photodetector signals from each of the four divided surfaces of the image plane of the focus detector. A focus error curve similar to the focus error curve shown in FIGS. 2A and 2B can be obtained.

  The processor 88 receives information from both the spectral sensitivity detection system 54 and the focus detector 74. From the spectral sensitivity detection system 54, the processor receives information related to variations in the intensity of the recombined test beam 26 and reference beam 28 over the wavelength range and relates to the axial position of the focal point 42 at the test surface 40. This is used for calculating the optical path length difference between the test arm 32 and the reference arm 34.

  From the focus detector, processor 88 receives information regarding the focus error. This focus error corresponds to the position where the test object surface 40 is displaced in the axial direction with respect to the focus 42. The range of focus error is related to the intended measurement area 82. For example, the polarity or magnitude of the focus error, or a combination of both polarity and magnitude, can be used to identify the intended measurement area 82. The optical path length measurement associated with the focus error within the intended measurement area 82 is retained (ie, selected as valid data) and the optical path length measurement associated with the focus error outside the intended measurement area 82. Is preferably discarded. Thereby, only a clear measured value of the optical path length difference is retained.

  Although not shown, the test object surface 40 is preferably supported by a coordinate measurement stage. This stage moves or rotates the test target surface 40 relative to the focal point 42 so that other points of the test target surface 40 with respect to the reference position range can be measured. For example, the coordinate measurement stage can provide spatial information regarding position changes on two coordinate axes (eg, the X and Y coordinate axes), and the interferometer 10 can be coupled to a third coordinate axis (eg, Z axis) that is aligned with the optical axis 80. Information on position changes on the coordinate axes) can be provided. In general, the interferometer collects information regarding variations in the height of the test surface 40. Preferably, individual measurements are performed at a rate of several hundreds to about 1000 times per second, and all or a predetermined portion of the test target surface 40 is defined as a sequence of measurement points.

  The video imaging system 90 is connected via the dichroic beam splitter 36, and images the test target surface 40 during setup and measurement of the test target surface 40. The optical zoom lens 92 relays a size adjustable image of the test object surface 40 to a video CCD detector 94 (detector array). The focusing system 60 preferably operates in the visible spectrum and uses this focusing system to generate a visible target spot on the test object surface 40, with respect to the visible features and boundaries of the test object surface 40. Allow measurable points on 40 to be matched.

  Another partial coherence interferometer 100 is shown in FIG. Features corresponding to the partial coherence interferometer 10 shown in FIG. However, in contrast to the interferometer 10, the common light source 102 provides optical power to both the interferometer 100 and the focusing system 110 associated therewith. This light source 102 is preferably a superluminescent diode operating in the visible spectrum (eg, wavelength of about 680 nanometers). By operating in the visible spectrum, the measurement spot appears at the focal point 42 on the test object surface 40 so that it can be set up and the measurement points matched to visible features or boundaries on the test object surface 40.

  Since the superluminescent diode is partially polarized, the focusing system 110 can be coupled to the interferometer 100 via the polarizing beam splitter 104. A quarter-wave plate 106 is used connected to the polarizing beam splitter 104, and the returned measurement beam 14 is reflected as a focus detection beam 64 along the focusing arm 112 of the focusing system 110. The polarizing beam splitter 104 increases efficiency and reduces feedback to the light source 102. Although the common light source 102 is shared, both the interferometer 100 and the focusing system 110 operate in the same manner as the interferometer and the focusing system in the previous embodiment.

  Further, instead of the polarizing beam splitter 104, a partially reflective or partially transmissive beam splitter can be used. In this alternative beam splitter, preferably transmission is emphasized in order to conserve more power of the initial measurement beam 14. On the other hand, the power of the focus detection beam 64 collected by the focus detector 74 is partially sacrificed.

  The embodiment of FIGS. 1 and 3 shows how the collimated test beam 26 and / or focus detection beams 64, 14 enter the shared objective lens 38. If the shared objective lens 38 is part of a finite conjugate video imaging system (i.e. it is not infinity corrected and therefore requires the incidence of a diverging beam on the dichroic beam splitter 36), then the optical design Those skilled in the art will recognize that two polarization beamsplitter 24 (FIG. 1) and dichroic beam splitter 36, 50/50 beam splitter 22 (FIG. 3) and dichroic beam splitter 36, or elsewhere Appropriate lens elements could be added to tie the optics.

  The measurement spot appears on the test surface because at least one of the measurement beam and the focus detection beam is in the visible spectrum, but in order to perform its main function, either or both of these beams are not visible in the infrared region. It may be used within the region. The partial coherence interferometer 10 is accommodated together with the focusing system 60, may be a stand-alone sensor, and these components may be separately accommodated or modularized. Although the partial coherence interferometer of FIGS. 1 and 3 is configured as a Michelson interferometer, the interferometer may take the form of other known interferometers.

  The above embodiments are merely introduced as examples showing how the present invention can be implemented. Those skilled in the art will appreciate that modifications, substitutions, and other changes can be made in accordance with the present invention to accommodate a variety of different applications.

Claims (20)

A partial coherence interferometer of the type employing a measurement beam having a predetermined spectral bandwidth ,
A test arm for sending the test beam portion of the measurement beam along a path encountering the test surface ;
A reference arm for sending a reference beam portion of the measurement beam along a path encountering the reference plane ;
A spectral sensitivity detection system coupled to the test arm and the reference arm;
A focus detection system calibrated with the interferometer,
The test arm saw including a focusing optic for collecting the reflected portion of the test beam from the test surface of the test beam is focused onto the test surface,
The spectral sensitivity detection system monitors interference as a function of wavelength over a range of measurement beam spectral bandwidth as a measure of the unsigned path length difference between the test arm and the reference arm ;
The focus detection system distinguishes between an optical path length difference between the test arm and the reference arm that is significantly greater than zero and an optical path length difference between the test arm and the reference arm that is significantly less than zero. In order to solve the ambiguity associated with the measurement of the unsigned optical path length difference between the test arm and the reference arm ,
The focus detection system includes a focus detection arm that transmits light reflected from the test surface through the focusing optical element to a focus detector;
The focus detection system is calibrated with the interferometer, which calibrates the focus of light passing through the focusing optical element from a zero state where the optical path length of the test arm and the optical path length of the reference arm are equal. An interferometer, characterized by being performed by positioning with a predetermined offset. The focus detection system is configured such that the optical path length difference between the test arm and the reference arm is significantly different from the position of the test surface where the optical path length difference between the test arm and the reference arm is significantly greater than zero. The interferometer according to claim 1 , wherein the interferometer is distinguished from a position of the test surface that is smaller than zero. Preferably, the focus detection system also distinguishes between an optical path length difference close to zero and an optical path length difference of any sign that is significantly larger or smaller than zero. The interferometer according to claim 2 . A processor further comprising: (a) a measurement from the spectral sensitivity detection system related to an unsigned optical path length difference between the test arm and the reference arm; and (b) the test surface is the optical path length difference between the test and reference arms is what is in the larger position than zero, indicating there to be small position, and wherein the receiving the information from the focus detection system, according to claim 2 The interferometer described in 1. The interferometer according to claim 2 , characterized in that the light reflected from the test surface, through the focusing optical element and to the focus detector is part of the measurement beam. A first light source for generating the measurement beam; and a second light source for generating a focus detection beam, the focus detection beam being passed through the focusing optical element by the focus detection arm. Interferometer according to claim 2 , characterized in that it is sent to the surface and from the test surface through the focusing optical element to the focus detector. The interferometer according to claim 6 , wherein a display wavelength of the focus detection beam is in a visible wavelength range, and a display wavelength of the measurement beam is in an invisible wavelength range. The interferometer has a predetermined measuring range, wherein the predetermined offset is half of the predetermined measuring range of at least the interferometer, the interferometer of claim 1. The difference in optical path length when the test beam of the measuring beam having a predetermined spectral bandwidth and the reference beam portion passes through a partial coherence interferometry be measured by monitoring the interference variation over a range of wavelengths of the combined beam There,
A test arm and a reference arm for sending the test beam portion and the reference beam portion of the measurement beam from a light source, and a focus detection system;
The test arm includes a focusing optical element for focusing the test beam on the test surface and collecting a reflected portion of the test beam from the test surface;
The focus detection system includes a focus detection arm that sends light reflected from the test surface through the focusing optical element to a focus detector;
The focal point of the focusing optical element is associated with a predetermined optical path length difference between the test arm and the reference arm, so that the focus detection system is significantly shorter than the optical path length of the reference arm. It is possible to distinguish between the length and the optical path length of a significantly longer test arm,
The focus detection system includes a focus detection arm that transmits light reflected from the test surface through the focusing optical element to a focus detector;
The interferometer, wherein the focal point of the focusing optical element is offset from a zero state in which the optical path lengths of the test arm and the reference arm are equal. The focus detection system identifies a focus error range corresponding to an optical path length difference range between the test arm and the reference arm, and in the optical path length difference range, the optical path length of the test arm is the optical path length of the reference arm. The interferometer according to claim 9 , characterized in that it is either significantly longer or significantly shorter. The optical path length difference range between the identified test arm and the reference arm does not include an optical path length difference close to a zero state in which the optical path lengths of the test arm and the reference arm are equal. The interferometer according to claim 10 . A processor, wherein the processor is configured to select an interferometric measurement of the optical path length difference between the test arm and the reference arm associated with the identified focus error range; The interferometer according to claim 11 . The interferometer according to claim 9 , characterized in that the light reflected from the test surface and passing through the focusing optical element to a focus detector is part of the test beam from the light source. The light source for generating the measurement beam is a first light source of two light sources, and a second light source generates a focus detection beam, which is detected by the focus detection arm. The interferometer of claim 9 , wherein the interferometer is sent through the focusing optical element to the test surface and from the test surface through the focusing optical element to the focus detector. The interferometer according to claim 14 , wherein a display wavelength of the focus detection beam is in a visible wavelength range, and a display wavelength of the measurement beam is in an invisible wavelength range. A method for resolving measurement ambiguity of partial coherence interferometry based on an unsigned optical path length difference between a test beam and a reference beam,
(A) generating a measurement beam having a predetermined spectral bandwidth;
(B) separating the measurement beam into a test beam propagating along the test arm and a reference beam propagating along the reference arm;
(C) directing the test beam along the test arm through the focusing optical element to the test surface and through the focusing optical element back from the test surface;
(D) recombining the test beam and the reference beam;
(E) monitoring a change in interference over a range of wavelengths of the recombined test beam and reference beam as a measurement of an unsigned optical path length difference between the test arm and the reference arm;
(F) sending the light reflected from the test surface through the focusing optical element to a focus detector for measuring a focus error;
(G) identifying the optical path length of the test arm, which is significantly shorter than the optical path length of the reference arm, and one that is significantly longer based on the measured focus error;
As a step further performed before the steps (a) to (g),
(H) the focus of the focusing optic, a method which comprises the step of positioning at a position offset from the zero state and the optical path length of the test and reference arms are equal. In the identifying step, the position of the test surface where the optical path length difference between the test arm and the reference arm is significantly greater than zero, and the optical path length difference between the test arm and the reference arm is more significant than zero. The method of claim 16 including identifying the position of the test surface to be smaller. 18. The method of claim 17 , wherein the step of identifying includes identifying an optical path length difference near zero and an optical path length difference of any sign that is significantly greater than or significantly less than zero. The method described. Identifying a focus error range corresponding to an optical path length difference range between the test arm and the reference arm, wherein the optical path length of the test arm is greater than the optical path length of the reference arm in the optical path length difference range. 17. A method according to claim 16 , characterized in that it is either significantly longer or significantly shorter. 20. The method of claim 19 , comprising selecting a monitored interference measurement of the optical path length difference between the test arm and the reference arm associated with the identified focus error range.